1. Field of the Invention
The present invention generally relates to a drive power transmission apparatus disconnectably connecting a first rotating element on the side of a drive power source with a second rotating element on the side of a drive-power-receiving unit for transmitting a drive power from the first rotating element to the second rotating element. More particularly, the present invention relates to a drive power transmission apparatus, not exclusively, but preferably, arranged between a vehicle engine and a vehicle auxiliary unit, such as a refrigerant compressor of a vehicle refrigerating system, and accommodating therein a transmission-interrupting mechanism for interrupting the transmission of a drive power from the vehicle engine to the auxiliary unit to stop the operation of the auxiliary unit when an excessive load appears in the auxiliary unit due to an unpredictable trouble of the auxiliary unit, in order to eventually prevent the excessive load being transmitted to the vehicle engine.
2. Description of the Related Art
The pending U.S. patent application Ser. No. 09/208,383 assigned to the Assignee of the present application discloses a drive power transmission apparatus with a transmission interrupting means. The disclosed drive power transmission apparatus includes a power transmission-pulley-assembly 100 having a torque limiter, as shown in the attached FIGS. 8 through 12.
As shown in FIGS. 8 and 9, the power transmission-pulley-assembly 100 includes a rotor element 101 rotatably supported on a front housing 201 of a refrigerant compressor and having an axis of rotation corresponding to an axis "L" shown in FIG. 8 about which the power transmission-pulley-assembly 100 rotates. The rotor element 100 is operatively connected to a vehicle engine 202 via a transmission belt 203 wound around both the rotor element 101 and a pulley mounted on an output shaft of the vehicle engine.
The rotor element 101 is provided with an annularly extending inner cavity 101a and spring catches 102 formed therein to be arranged in the inner cavity 101a. Each of the spring catches 102 has a portion thereof projecting from the inner cavity 101a and an engaging recess 103 recessed in an end face of the spring catch 102, i.e., in a front face thereof in a predetermined rotating direction of the power transmission-pulley-assembly 100, indicated by an arrow in FIG. 9. The engaging recess 103 is formed to extend from the inside of the inner cavity 101a toward the outside of the inner cavity 101a. The engaging recess 103 of each spring catch 102 opens in a radial direction of the power transmission-pulley-assembly 100 and is closed by walls in a direction parallel to the axis "L". The engaging recess 103 has an engaging end face 103a formed in the inner cavity 101a as a face through which a power is transmitted from the rotor element 101 to a later-described spiral spring 104. The engaging end face 103a of the engaging recess 103 is arranged to direct forwardly with respect to the rotating direction of the power transmission-pulley-assembly 100. One of the walls of the engaging recess 103 is formed as a blocking face 103b facing toward the inside of the inner cavity 101a, i.e., facing in a direction from the left to right hand along the axis "L" in FIG. 8.
The power transmission-pulley-assembly 100 is further provided with a pair of spiral springs 104 arranged between the rotor element 101 and a drive shaft 204 of the refrigerant compressor. Each spiral spring 104 has an outer end 104a which is received in the engaging recess 103 of the spring catch 102 so as to be abutted against the engaging end face 103a thereof. The inner end 104b of the spiral spring 104 is fixed to the drive shaft 204 in a region outside the inner cavity 101a of the rotor element 101.
The spiral springs 104 shown in FIG. 11 are placed in a free condition where the springs 104 recover their basic position lying in a flat plane which is vertical to the axis "L" and is located in front of the rotor element 101 in a direction of the axis "L", so that the outer end 104a thereof is detached from the spring catch 102. Thus, when the power transmission-pulley-assembly 100 is assembled on the front end of the refrigerant compressor, the spiral spring 104 is elastically deformed in the direction of the axis "L" so that the outer end 104a thereof is moved rearward from the above-mentioned flat plane with respect to the inner end 104b so as to be engaged in the engaging recess 103 of the spring catch 102, as shown in FIG. 8. When the outer end 104a of the spiral spring 104 is engaged in the engaging recess 103 of the spring catch 102, the outer end 104a is elastically urged frontward in a direction along the axis "L" to come into contact with the blocking face 103b, so that an elastic force is produced and stored in the spiral spring 104.
A disconnecting plate 105 of the power transmission-pulley-assembly 100 is fixed to the drive shaft 204 and arranged axially in front of the spiral spring 104 along the axis "L". The disconnecting plate 105 is provided with a pair of circularly elongated projections 105a formed therein and functioning as a releasing means for permitting the outer end 104a of the spiral spring 104 to be disengaged from the engaging recess 103 of the spring catch 102. The disconnecting plate 105 is fixedly mounted on the drive shaft 204 so that each of the pair of circularly elongated projections 105a is shifted circumferentially in the rotating direction of the transmission-pulley-assembly 100 with respect to the corresponding one of the pair of spring catches 102.
A drive power from the vehicle engine 202 is transmitted to the drive shaft 204 via the transmission belt 203, the rotor element 101, the engaging end face 103a of the spring catch 102, and the spiral spring 104 having the outer and inner ends 104a and 104b. As soon as the drive power is transmitted to the drive shaft 204 to rotate it in the predetermined rotating direction shown in FIG. 9, the drive shaft 204 is subjected to a load torque in a direction reverse to the predetermined rotating direction thereof. The load torque applied to the drive shaft 204 causes torsion of the spiral spring 104, so that the rotor element 101 is relatively shifted circumferentially with respect to the drive shaft 204 in a direction corresponding to the predetermined rotating direction of the drive shaft 204. Thus, each of the spring catches 102 integral with the rotor element 101 approaches the corresponding releasing projection 105a of the disconnecting plate 105 fixed to the drive shaft 204.
When the above-mentioned load torque is smaller than a predetermined limiting torque, an amount of torsion of the spiral spring 104 is kept small and accordingly, the relative amount of shift between the rotor element 101 and the drive shaft 204 is also kept small. Thus, although the releasing projections 105a of the disconnecting plate 105 are shifted to positions close to or in contact with the outer ends 104a of the spiral springs 104, the contacting force acting between the outer ends 104a of the spiral springs 104 and the releasing projections 105a is not large enough to release a mechanical engagement of the outer ends 104a of the spiral springs 104 with the rotor element 101 via the engaging end face 103a of the spring catch 102. Therefore, the outer ends 104a of the spiral springs 104 are engaged in the engaging recesses 103 of the spring catch 102 so that the outer ends 104a are kept in touch with the engaging end face 103a. Accordingly, the transmission of the drive power from the vehicle engine 202 to the drive shaft 204 continues. A change in the load torque can be absorbed by the torsion of the spiral springs 104 so long as the changed load torque exceeds the predetermined limiting torque.
On the other hand, when the load torque excessively increases to exceed the predetermined limiting torque due to an unpredictable cause appearing in the refrigerant compressor, the amount of torsion of the spiral spring 104 is increased to extend the relative shift between the rotor element 101 and the drive shaft 204 in the rotating direction of the drive shaft 204. Thus, the releasing projections 105a of the disconnecting plate 105 come into strong contact with the outer ends 104a of the spiral springs 104 to apply a large pressing force to the outer ends 104a of the spiral springs 104. Thus, the engaging end faces 103a of the spring catches 102, which have an inclination from a radial direction (see FIG. 10), cause an increase in a radially inward force applied by the releasing projections 105a to the outer ends 104a of the spiral springs 104 so that the outer ends 104a of the spiral springs 104 are moved radially inwardly by the guidance of the inclined engaging end faces 103a until the outer ends 104a are disengaged from the engaging end faces 103a of the spring catches 102, as best shown in FIG. 10. As a result, the elastic force stored in the spiral springs 104 is removed. Namely, the spiral springs 104 are moved away and released from the engagement with spring catches 102 of the rotor element 101 and accordingly, the spiral springs 104 recover the free condition shown in FIG. 11 in which they lie in the flat plane perpendicular to the axis "L". Therefore, the outer ends 104a of the spiral springs 104 are detached from the engaging end faces 103a of the spring catches 102 in the direction toward the disconnecting plate 105. Thus, the spiral springs 104 are completely disconnected from the rotor element 101, so that the transmission of the drive power from the side of the rotor element 101 of the power transmission-pulley-assembly 100 to the drive shaft 204 of the side of the refrigerant compressor is interrupted to prevent the excessive load torque from being transmitted from the side of the refrigerant compressor to the side of the vehicle engine 202.
Nevertheless, in the described conventional power transmission-pulley-assembly 100, each of the spiral springs 104 having its maximum radius R1 (a radial distance between the axis "L" and an outermost edge portion of the outer end 104a located farthest from the axis "L") which is larger than the radius R2 of the outer wall portion of the annular rotor cavity 101a of the rotor element 101 (see FIG. 11) is intendedly or unintendedly used depending on the design requirement of the power transmission-pulley-assembly 100. Therefore, the outer diameter of the spiral springs 104 must be forcedly and elastically reduced when the spiral springs 104 are assembled in the rotor cavity 101a of the rotor element 101 in a manner such that the outer ends 104a of the spiral springs 104 are in touch with the engaging end faces 103a of the spring catches 102 of the rotor element 101. Therefore, when the load torque applied by the refrigerant compressor increases to exceed the predetermined limiting torque to resultingly cause an increase in the radially inward force applied to the spiral springs 104 from the releasing projections 105a of the disconnecting plate 105, a movement of the spiral springs 104 to come out of the rotor cavity 101a of the rotor element 101 occurs. Namely, the reduced outer diameter of the spiral springs 104 is elastically allowed to recover the initial outer diameter while permitting the outer ends 104a of the spiral springs 104 to be moved axially forward by the guidance of the inner wall surface 101b of the rotor element 101, and finally the outer ends 104a of the spiral springs 104 are moved away and separated from the rotor cavity 101a of the rotor element 101.
Nevertheless, as shown in FIGS. 12A and 12B, the conventional spiral springs 104 must encounter the defects described hereinbelow. Namely, when the spiral springs 104 are assembled in the engaging recesses 103 of the spring catches 102 of the rotor element 101, they might either fail to produce a sufficient elastic force in the axially forward direction to cause a disengagement thereof from the spring catches 102 or may generate a large frictional force in a portion contacting with the inner wall surface 101b of the rotor cavity 101a. Thus, the outer ends 104a of the spiral springs 104 cannot be completely moved away from the rotor cavity 101a of the rotor element 101 even when the load torque exceeds the predetermined limiting torque. As a result, the outer ends 104a of the spiral springs 104 are still left in touch with and frictionally slide along the inner wall surface 101b of the rotor cavity 101a to generate noise and vibration during the relative shifting of the rotor element 101 in the circumferential direction with respect to the drive shaft 204. Further, each of the outer ends 104a of the spiral springs 104 might come into re-engagement with the engaging recess 103 of the spring catch 102 which is different from the initially engaged spring catch 102, due to the relative shifting of the rotor element 101 in the circumferential direction with respect to the drive shaft 204, and therefore, the power transmission-pulley-assembly 100 cannot surely prevent an excessive load torque from being transmitted from the side of the refrigerant compressor to the side of the vehicle engine 202.